The subject matter disclosed herein relates to processing of data that corresponds with operating parameters for a valve assembly, with particular discussion about methods that use this data to quantify movement of the valve stem due to stick-slip to distinguish the root cause and/or contributing factor of cycling on the valve assembly.
Process lines may include many varieties of flow controls. These process lines typically transfer fluids for use in the chemical industry, refining industry, oil & gas recovery industry, and the like. Examples of the flow controls include pneumatic and electronic valve assemblies (collectively, “valve assemblies”) that regulate a flow of process fluid (e.g., gas and liquid). In conventional implementation, these valve assemblies have a number of components that work together to regulate flow of process fluid into and/or out of the process line. These components include a closure member, a seat, a valve stem, and an actuator. Examples of the closure member may embody a plug, ball, butterfly valve, and/or like implement that can contact the seat to prevent flow. In common construction, the actuator couples with the closure member (via the valve stem). The valve assembly may also incorporate a valve positioner with electrical and/or electro-pneumatic components. During operation, the valve positioner receives control signals from a controller that is part of a process control system (also “distributed control system” or “DCS”). These control signals define operating parameters for the valve assembly, namely, a position for the closure member relative to the seat. In response to the control signal, the valve positioner delivers a pneumatic signal that regulates instrument gas to pressurize the actuator in order to regulate this position.
Cycling of a valve assembly can have many root causes. At the process level, the process control system may provide setpoint instructions that oscillate back-and-forth between various levels. The valve assembly will, in turn, modulate the position of the valve stem in response to these instructions. From the device level, the valve assembly may suffer from improperly tuned components (e.g., valve positioner) and/or have some type of physical problem that can cause cycling to occur on the valve assembly. Improper tuning (and calibration) of the valve positioner, for example, may translate the setpoint instructions into the pneumatic signal incorrectly and, thus, cause the valve stem to move to improper positions. On the other hand, physical problems can frustrate operation of the mechanical components on the valve assembly. Stick-slip describes a condition, for example, that results from high static friction and/or low kinetic friction between the valve stem and the valve packing material that surrounds the valve stem. This configuration can prevent leaks from inside the valve assembly. During operation, however, stick-slip can cause internal force, i.e., pressure on the closure member, to exceed normal operating levels in order to overcome the static friction that prevents movement of the valve stem. These internal forces, while desirable to induce movement of the valve stem, can cause the valve stem to overshoot its desired position.
The process control system will attempt to correct this error. In response to overshoot, for example, the process control system often issues instructions for the valve assembly to modulate the valve stem in the opposite direction. However, it is likely that stick-slip will disrupt the movement in the opposite direction. The resulting errors will again prompt intervention from the control system to move the valve stem in the other direction. This process repeats itself, resulting in oscillation or cycling of the valve stem that can disrupt the stability of the process line as well as cause unnecessary wear on the valve assembly.